1. Field of the Invention
The present invention relates to a coefficient of friction peak estimation apparatus for estimating the peak of a coefficient of friction in a coefficient of friction--slip rate characteristic, and relates to an anti-skid control system having the coefficient of friction peak estimation apparatus to control a hydraulic braking pressure in each wheel brake cylinder in response to a wheel speed detected when the peak of the coefficient of friction is estimated.
2. Description of the Related Arts
Various anti-skid control systems have been proposed heretofore to prevent a wheel from being locked during a braking operation of a vehicle. For example, Japanese Patent Laid-open Publication No.8-150918 discloses an anti-skid control system which is adapted to compare a wheel acceleration with a vehicle acceleration at a predetermined time, and adjust a pressure increasing rate of braking pressure, based upon the result of comparison. According to that publication, in view of a prior anti-skid control system as disclosed in Japanese Patent Laid-open Publication No.4-345567, which was aimed to provide an appropriate pressure increasing rate in accordance with the slip rate under such a condition as the coefficient of friction of a road (abbreviated as a road .mu.) or the like is constant, thereby to achieve an ideal braking force control, it was proposed to solve such a problem that the ideal braking force control could not necessarily be achieved by that prior anti-skid control system, when the condition such as the coefficient of friction of the road was varied, so that it is aimed to control the braking force in accordance with the coefficient of friction of the road.
Furthermore, in Japanese Patent Laid-open Publication No.8-295223, there is disclosed a vehicle speed estimation apparatus which is provided with means for determining whether a changing point of a property in a road coefficient of friction--slip rate characteristic was crossed, and adapted to change the estimation of a vehicle speed into the estimation of the vehicle speed on the basis of an estimated vehicle deceleration calculated by vehicle speed estimation means, and also disclosed is the anti-skid brake system having the vehicle speed estimation apparatus.
According to the vehicle speed estimation apparatus as disclosed in the above Japanese Patent Laid-open Publication No.8-295223, it is determined on the basis of a varying rate (.DELTA.dVw) of a wheel acceleration as shown in FIGS. 8 and 9 of that publication, whether the changing point of the property (corresponding to the peak) of the coefficient of friction--slip rate characteristic was crossed. According to that apparatus, in the case where the variation around the peak in the road coefficient of friction--slip rate characteristic is large, the peak can be identified. In the case where the variation around the peak is small, however, the varying rate (.DELTA.dVw) of the wheel acceleration is necessarily small, so that it will be difficult to distinguish the peak. This is resulted from not only road conditions, but also tire properties. For example, the variation around the peak in case of a tire without studs (i.e., stud-less tire) is so small that it will be difficult to distinguish the peak.
In the meantime, the aforementioned Japanese Patent Laid-open Publication No.8-150918 never discloses the estimation of the peak in the coefficient of friction--slip rate characteristic, nor suggests it.